Home Insulation

Gerald Cole answers some common questions about domestic insulation

Glass wool insulation batts being installed in a full-fill cavity wall.
Not so long ago home insulation consisted of a few inches of fibre glass or mineral wool loose laid on the loft floor, and a set of lined curtains. But that was before central heating, double glazing, ever rising fuel prices and climate change.

Today’s Building Regulations take account of all those things to make new homes more energy efficient and produce less carbon dioxide than ever before. The result has been an explosion in new forms of insulation and construction techniques designed to meet these ever-increasing demands.

To the newcomer, however, it can all seem a little confusing. But understanding how insulation works and what options are available will make a big difference not just to the comfort and fuel bills of your new home but to the way it is designed and built. Here, then, are answers to some common questions.

Where does the insulation go?

Thinsulex RafterFit

In the roof, external walls and floor, ideally forming a continuous barrier to maximise its effect. Roof insulation is fitted either between the rafters, below them or on top, a combination of these or all three together, depending on the specification.

Multifoil from Thinsulex is applied to the rafters


Wall insulation typically goes inside the cavity, either filling it completely or partially. It can also be applied externally in the case of a new single-leaf wall, or on an existing wall in a renovation or conversion.

ROCKWOOL sound insulation

Rockwool sound insulation installed to internal walls

Floor insulation is fitted either below a solid concrete foundation slab (though above the damp proof membrane), or above the slab, after which it’s topped by a finishing screed or a ‘floating’ chipboard floor. With a suspended timber floor, insulation sits between the joists.


How much insulation do you need?

That depends on the performance of the insulation and the amount of heat you’re willing to lose. Most of us would prefer to lose as little as possible, but the maximum allowable amount is indicated in the Building Regulations. Or rather it was. The latest version now requires a calculation known as the Standard Assessment Procedure, or SAP, which provides an assessment of the house’s total energy needs and predicted carbon dioxide emissions. As well as heat loss, SAP takes into account a host of other factors, including heat gained from sunlight and the efficiency of heating, lighting and ventilation systems.

All this results in an Energy Performance Certificate (EPC) for the property. That said, the regulations also recommend maximum U-values for walls, roofs and floors.

What’s a U-value?

A U-value is a measure of the rate at which heat flows through a material – any material from a brick wall to a sheet of insulation. It actually describes the amount of heat in watts passing through every square metre of the material for each degree of temperature difference (in Centigrade or Kelvin) between the inside and outside of a building – written as W/m2K. But all you really need to know is that the lower the number the slower the heat flow, so a high-performing insulation will have a lower U-value than a less effective one.

All you really need to know is that the lower the number the slower the heat flow

Currently the maximum recommended U-values for new builds are 0.16 for walls and 0.11 for roofs and floors in England and Wales, and in Scotland 0.15 for walls, 0.13 for floors and 0.10 for roofs. U-values for extensions and renovations are higher because there is less control over the fabric of existing buildings.

In a new build, then, your goal is to match these U-values to ensure a good degree of internal comfort. In practice, you may wish to exceed them, either to be more energy efficient (and lower fuel bills even further) or because Building Control requires you to make up for another aspect of your design which doesn’t meet the regulations. But this is really a problem for your architect or designer to solve – in consultation with you.

So how do you choose which insulation?

A simple rule is that the cheaper and poorer performing the insulation, the more of it you will need. In other words, your walls and roof and, to a lesser extent, floor must be designed to accommodate the greater volume of insulation.

The most common construction method in the UK is brick and block: an inner supporting wall of concrete blocks and an outer wall of brick or stone, separated typically by a 100mm-wide cavity into which insulation is inserted.

Denby Dale Passivhaus the UK's first cavity wall Passivhaus courtesy of Green Building Store

However, with the Denby Dale house (above), the first conventionally built Passivhaus home in the UK, the cavity had to be 300mm (about a foot) wide to accommodate the required thickness of mineral wool. This resulted in walls with a total depth of 380mm (around 15in).

Denby House - 300mm cavity filled with ROCKWOOLThe Denby Dale house, Britain’s first traditionally built Passivhaus, needed a 300mm-wide cavity to meet the required amount of Rockwool insulation.

On a large plot this may not be a problem. On a small or restricted plot it could result in a significant reduction to the internal floor space.

There are two solutions to this. Choose a slimmer, better performing but more expensive insulation, or change your method of construction. Fortunately there are plenty of choices which can squeeze more insulation into a given space.

The best known is timber frame, where the inner wall is an open framework, able to accommodate insulation. Timber is also inherently more thermally efficient than brick or block.

Another alternative is to use lightweight, aircrete blocks for the inner wall. Aircrete is filled with tiny air bubbles, again more thermally efficient than dense concrete.

More radically, there are systems where the insulation actually provides the supporting structure. Insulating concrete formwork (ICF) consists of hollow blocks of expanded polystyrene which are assembled by hand, then filled with concrete which is left to set, forming a permanent structure.

Structural insulated panels (SIPs) are ceiling-high panels of rigid foam insulation sandwiched between sheets of oriented strand board. To erect a wall one panel is simply glued and nailed to another until a complete storey is built. Both SIPs and ICF maximise insulation because they form a continuous airtight barrier.

See Guide to Structural Insulated Panels

How do you make insulation effective?

No matter how high-performing insulation is in principle it will not be very effective if it leaks. In fact, according to a recent study, a one millimetre gap between sheets of insulation can reduce its efficiency by 40 per cent.

The solution is to minimise leaks by making the house as airtight as possible. One of the most effective methods is to apply the insulation externally, enclosing the house in an all-encompassing ‘tea cosy’ so that any gaps or deficiencies are easy to spot. But this is typically reserved for upgrading existing and architecturally unremarkable properties.

Southcliffe Tower renovation - Baumit wall insulation applied externally

Baumit wall insulation being applied to a multi-residential building. External wall insulation is well suited to old single-brick buildings, being an effective way of treating condensation when insulation is required on one or two walls.

Airtightness is now included in the Building Regulations which stipulate the maximum volume of air allowed to leak through a square metre of a home’s exterior. It’s called the Air Permeability figure. It’s actually quite generous – or lax, depending on your point of view.

It’s essential, however, that a well-insulated home with good levels of airtightness also needs to be well ventilated. Otherwise, mould can spread and overheating occur in hot weather, as many new homeowners discovered this summer.

Currently the most efficient solution is to incorporate a whole house ventilation system. Here, a fan runs continuously, drawing warm, moist air out of the kitchen and bathrooms, and replacing it with fresh air from outside, all through a system of ducts. Separate fans can also be used, and there are ductless systems where a central fan maintains a slight but constant raised pressure throughout the house, squeezing out moist air.

As homes become better insulated, heat can also escape through thermal or cold bridges

But leaks don’t necessarily require gaps. As homes become better insulated, heat can also escape through thermal or cold bridges. These are points where parts of the building fabric bypass or penetrate the insulation. Common culprits are window reveals, pipes, cables and metal wall ties in a cavity wall. The areas involved may seem negligible, but in a super-insulated house the resultant heat loss can be as high as 30 per cent.

Building homes with high attention to detail is a major challenge for traditional methods of construction

Building homes that demand such a high degree of attention to detail is a major challenge for traditional methods of construction, but it’s one the industry is going to have to meet as standards continue to rise.


Types of insulation

Glass wool

Glass wool insulationMade from sand and recycled glass, it’s available in rolls or stiffened (known as ‘wicked’) slabs or batts, which are easier to use in cavity insulation. It’s lightweight, recyclable and good for both thermal and acoustic insulation. It becomes ineffective, however, when wet, so should not be used in exposed situations. It also requires significant thickness to match the efficiency of rigid foam insulation. Manufacturers: Knauf, Isover.


Mineral wool

Mineral wool insulationMade from molten rock, it shares many of glass wool’s characteristics and comes in similar forms but is exceptionally fire and water resistant, so can be used in exposed conditions. Manufacturers: Rockwool, Knauf.


Expanded polystyrene (EPS)

expanded polystyrene insulationEquivalent in performance and price to glass and mineral wool, it is a lightweight, rigid, durable board, unaffected by water, though classified as combustible. It is used for partial-fill cavities, underfloor and external wall insulation. Manufacturers: Jablite, Kay-Metzeler.


Extruded polystyrene (XPS)

extruded polystyrene insulationA denser, stronger form of polystyrene, its main selling point is its resistance to water penetration, making it popular for insulating foundations. Manufacturers: Kingspan, Dow, Knauf.


Polyisocyanurate (PIR) and phenolic insulation

polyisocyanurate insulationThe most efficient forms of insulation, these rigid foam boards are backed with aluminium foil which acts as protection, a moisture barrier and a reflective heat barrier. Essentially they provide twice the efficiency of cheaper insulation, but at three times the price. Manufacturers: Kingspan, Celotex, Xtratherm.



multifoil insulationUnlike other forms of insulation which use pockets of air or gas to slow the passage of heat, multifoils use shiny material to reflect heat back. Typically they consist of layers of aluminium foil alternating with layers of more conventional insulation. They are, however, very thin, making them ideal for restricted spaces, such as loft conversions. To function properly they need a small air space on either side of the application. Manufacturers: Actis, TLX Insulation, YBS Insulation.


Sheep’s wool

sheep's wool insulationA natural product which has the same performance as mineral wool but is also hygroscopic. It is able to absorb and give up moisture and so regulate humidity. It will also absorb and neutralise airborne toxins, so good for allergy sufferers. Its main drawback is price: around five times that of mineral wool. Manufacturers: Thermafleece, Black Mountain.


Other eco products

recycled newspaper insulationThese include hemp and flax, recycled newspaper and wood fibre, made from wood waste. Hemp, flax and recycled paper are hygroscopic. All four are also breathable – allowing water vapour to pass through without losing their insulation value. These materials are most commonly used in timber-frame homes built without an enclosing and airtight vapour barrier. Manufacturers: Thermafleece, Warmcel, Pavatex.



aerogel insulationOriginally developed by NASA, this lightweight, low-density material, made from silica gel, is normally bonded to plasterboard, magnesium oxide board or chipboard. It can match the best performing conventional insulations in a fraction of their thickness, making it ideal for situations where space is limited, particularly renovations and conversions. Its high cost, however, restricts its use. Manufacturer: The Proctor Group.


Vacuum insulated panels (VIPs)

Also highly efficient, these thin panels consist of an airtight reflective foil enclosing a rigid interior from which much of the air has been extracted. Effectively it’s a vacuum flask flattened into a panel. Drawbacks are its vulnerability, high price and the fact that only standard sizes are available. Manufacturers: Kingspan, Kevothermal.

Vacuum insulation panels (VIPs) consist of foil-sealed rigid core from which much of the air has been removed.

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